1. Field of the Invention
This invention relates to a composite compact material and a process for the production of the same and more particularly, it is concerned with a composite compact tool comprising a diamond or high pressure form boron nitride composite compact and a substrate bonded to the composite compact and a process for the production of the same.
2. Description of the Prior Art
Lately, hard sintered body or compacts of diamond or boron nitride have been developed and put to practical use. The hard compacts have generally a composite structure in which compact layer 1 is bonded to base material 2 as shown in FIG. 1 and which is fixed to a tool substrate by brazing. Such a composite compact has been used for not only brazed cutting tools or throw-away inserts but also rotary cutting tools such as end mills and boring cutters.
The diamond or BN compacts, obtained by sintering diamond or BN fine powder with a binder of an iron group metal under an ultra-high pressure at a high temperature, have a much higher wear resistance than the prior art cemented carbides and are thus suitable for use in cutting tools, wire drawing dies and drill bits.
These compacts have the excellent properties as a tool material, but an ultra-high pressure apparatus is required for the production thereof. Thus, their size and shape are more restricted as compared with those of the cemented carbides.
In general, a tool component is a disc-shaped composite compact as shown in FIG. 2, which comprises diamond or boron nitride compact 1, base 2 of cemented carbide for supporting compact 1 and intermediate joint layer 3 as described in Japanese Patent Application No. 129127/1979. This composite compact is bonded to a steel holder by brazing as it is disc-shaped or after cutting in a suitable shape to form a cutter, for example. However, it is found that when the diamond or boron nitride compact is heated at higher than about 700.degree. C. for a certain period of time during the brazing, the properties are degraded. Thus, the brazing is carried out using a silver braze having a low melting point. For the general use such as cutting tools, this brazing method using a low melting point braze is sufficient if the cutting tool is used under such a condition that cutting stress is relatively small, but in the case of applying the compact to a drill bit for drilling rocks, this brazing method is not sufficient.
In a drill bit, a number of composite compacts as shown in FIG. 2 are mounted in a bit crown as shown in FIG. 5 and used as cutters, which is disclosed in, for example, U.S. Pat. No. 4,098,362. When drilling of rocks is carried out using a drill bit fabricated by mounting composite compacts as shown in FIG. 2 in a bit crown with a low melting point braze having a melting point of lower than 700.degree. C., relatively soft rocks such as sandstone can be drilled without problem, but in the case of drilling hard rocks, some problems are encountered that the compact cutter falls off from the brazed part and the brazed part is moved. Silver braze (e.g., JIS B Ag-1) commonly used as a low melting point brazing material has a shearing strength of at most 20 kg/mm.sup.2 at room temperature, which strength is markedly lowered at high temperatures. A drill bit meets with a large fluctuation of stress because to the drilling stress added to the edge is large and rocks are scarcely uniform. Furthermore, even if a drilling fluid such as mud is used, not only the temperature of the edge, but also that of the bit itself rise in the case of drilling a high depth stratum. Depending upon the kind of a stratum mud cannot be used sometimes.
When a diamond compact is applied to a drill bit, therefore, it is very important how to fix the compact as shown in FIG. 2 to a bit crown.
On the other hand, micro diameter drills of cemented carbides have been used for making holes in a printed circuit board in household electric appliances and electric measuring instruments. It is a difficult problem to make a hole with dimensional presicion and without forming burrs at the inlet or outlet of the hole, and making fine splits or seizures on the inner surface of the hole, in a printed circuit board consisting of a laminate of e.g. an epoxy resin and a thin copper plate.
Of late, composite hard compacts have widely been used and in particular, diamond compact tools have often been used for cutting non-ferrous alloys such as aluminum and plastics with a life prolonged several tens to several hundreds times as long as that of the commonly used cemented carbide tools. Accordingly, diamond compact tools are said to be very desirable materials as a tool for making a holes in printed circuit boards consisting of laminates of resins and copper.
A drill for making a hole in a printed circuit board is prepared by brazing and fixing a diamond compact blank to the end of the drill as shown in FIG. 4 in marrer similar to that of ordinary diamond compact tools, but is a high strength bond is hardly by each method because the drill diameter is generally less than 3 mm, more generally about 1 mm. That is, a silver braze having a melting point of about 650.degree. to 700.degree. C. is ordinarily used for brazing a diamond compact blank and this gives a brazing strength of only about 20 kg/mm.sup.2 in terms of shearing strength. By this strength, there is a very high possibility that the diamond compact blank may be separated from the joint part by the cutting torque during drilling. When the inventors have made a drill as shown in FIG. 4 and carried out a drilling test of a printed circuit board, it is found that the blank is missed by one hole.
There are a number of brazing materials and some brazing materials having a higher brazing strength than silver brazes. However, in general, high strength brazing materials have a higher melting point and this is a problem. Intrinsically diamond is a thermodynamically unstable material under atmospheric pressure and thus tends to be transformed into graphite when the temperature is raised under atmospheric pressure. In the diamond compact as set forth above, particularly, this graphitization takes place through the reaction with a binder metal used in the compact at a temperature above 700.degree. C. The boron nitride compact are also degraded at a temperature of above 700.degree. C. Therefore, a brazing material to be used should have a melting point of lower than 700.degree. C.